It is well known that it is very important to accurately set the sensitivity level, or threshold for receiving sensed signals, in an implantable pacemaker. Thus, in sensing cardiac signals, e.g., QRS, T, or P waves for use in controlling timing of pacemaker operations, it is important to be able to accept bonafide, or true signals, but block extraneous or noisy signals that may be detected at the lead electrodes. Many techniques have been developed for aiding in this task, as can be seen from the prior art. One technique is to filter the signals so as to enhance acceptance of only the type of waveform that is being monitored. Another technique is to use an adjustable "window" which allows the signal to be passed through the sense amplifier only around the time that the periodic signal is expected. But the most important technique is to set a threshold, or sensitivity level (also referred to herein as simply sensitivity), above which the input signal must rise before it is recognized. But a fixed sensitivity level is generally insufficient, since signal levels may change, rendering the fixed level inappropriate. Thus, if the level is too low, too much noise can pass through the sense circuitry; and if the level is too high, too many real signals can be lost because they don't pass the threshold, or sensitivity level.
There have been two primary approaches to solving the above sensitivity level problem. A first approach is to provide circuitry which sets the sensitivity based on observed peak values of the input signal. See, for example, U.S. Pat. No. 5,339,820. A second approach is to adjust the gain of the sense amplifier, i.e., increase the gain if the R wave peaks become lower, and decrease gain when such peaks become higher, to provide a substantially unchanging ratio of signal peak value to amplifier sensitivity. See, for example, U.S. Pat. Nos. 4,708,144; 4,880,004; and 4,903,699. However, these solutions do not provide for optimal adjustment of sensitivity on a beat-to-beat basis, i.e., they don't respond as quickly and accurately to changing conditions as desired. This is important in many situations, e.g., occurrences of ventricular tachycardia (VT) and ventricular fibrillation (VF); atrial fibrillation and atrial flutter.